This invention relates primarily to overhead electric hoists, but is equally advantageous for other types of apparatus where similar conditions exist. It is specifically concerned with apparatus driven by single phase, or polyphase alternating current induction motors.
Hoists driven by single phase motors are generally of the capacitor start induction run variety wherein the motor stator has two windings; one generally designated as the running winding, and the other the starting winding. A capacitor is connected in series with the starting winding as is a centrifugal switch. At standstill when power is applied, the switch contact is closed and both windings are energized whereupon the motor develops torque and the rotor of the motor accelerates. When it reaches approximately three quarters of its normal running speed, the centrifugal switch contact opens removing power from the starting winding and its series capacitor. The motor now has sufficient torque to continue accelerating to its normal running speed. It is characteristic of single phase motors of this type that they cannot be reversed while running, which is considered a necessary condition in hoist apparatus. In order to provide reverse while running capability, it has been general practice to provide a specially designed centrifugal switch, which, when the centrifugal mechanism actuates, a frictional connection is made between the switch contact mechanism and the rotor of the motor such that it reacts to the direction of motor rotation by opening one contact to de-energize the starting winding and at the same time closing a second contact through which the same winding can be re-energized with reverse polarity. When reverse polarity is thus applied, the motor develops torque in the opposite direction to its rotation, stops and accelerates in the reverse direction, whereupon the action of the centrifugal switch described above repeats itself.
The centrifugal switch, while adequate for the purpose of making the single phase capacitor start induction run motor reversible while running has several disadvantages. It is relatively costly to build and install and being mechanical in nature and frictionally engaged it is subject to wear and failure. The electrical contact for various reasons, such as erosion and contamination, can fail to perform either to close or open the circuit intended. Critical location and mounting requirements make maintenance of the switches difficult.
Since the start winding and its series capacitor are energized only during the very short time during which the motor is accelerating, it is general practice to use a relatively small and inexpensive electrolytic capacitor in series with the starting winding of the motor. These capacitors are subject to failure due to overheating if the motor is subjected to too many starts and/or reversals and are generally less reliable than other types.
Further, capacitor start motors as applied to hoists have had the characteristic of developing high starting torques, typically three times rated torque. Heretofore, high starting torque has been considered a desirable feature of hoist motors. High starting torques were necessary with many hoist designs using mechanical load brakes since these load brakes have a tendency to lock up under certain conditions of operation and require considerable excess motor torque to break them free. The high torque also gave fast acceleration such that a typical hoist with a capacity load reached full running speed in approximately 1/10 second. The use of high torque motors on hoists have a number of disadvantages. The hoist thus equipped is capable of lifting loads far in excess of the full rated capacity of the hoist. Hoisting loads above rated capacity is generally considered a very serious safety hazard. In recognition of this, it is becoming common practice to provide overload switches on such hoists to cut off power to the hoist when it is overloaded.
Further, the high starting torque makes control of the hoist more difficult. The rapid acceleration of the motor, especially in the lowering direction, makes it difficult to achieve small increments of motion necessary to place a load in a precise location. Further, the current drawn on starting by the typical single phase hoist motor is very high, being on the order of five times its normal full load running current. The large starting current often causes voltage drops on the supply line which may cause problems with other equipment on the same supply. Additionally, high starting torque of the drive motor tends to apply higher stresses to other parts of the apparatus and increase the wear thereof.
The use of polyphase alternating current induction motors eliminates a number of the above difficulties with switching capacitors and hence have found wide use for hoist motors. However, a number of problems still remain and are common with single phase hoist motors.
Electric hoists are generally used for handling heavy loads overhead, their safe usage is largely dependent on the effeciency and reliability of their braking systems. Early hoists using alternating current motors were generally equipped with two independent brakes, one of which was known as a mechanical load brake and the other a spring set electrically released friction brake generally referred to as a holding or motor brake. Each of these brake types were designed to stop and hold any load within the capacity of the hoist. Thus, the failure of one would not result in a free falling load.
The mechanical load brake is a device wherein friction surfaces are brought into engagement by means of torque derived from the suspended workload in a manner to retard and stop the descent of said workload. The frictional surfaces tend to be released from engagement by torque from the motor in the lowering direction. The design is made so that with the holding brake released and the motor driving downward, the load brake generates a frictional force to maintain the speed of the descending load at something under the no load speed of the motor. However, if the motor torque is then removed such as through a power supply failure, the brake would stop and hold the load. The load brake is disengaged during hoisting by a clutching mechanism.
While effective from a safety standpoint, the load brake has many disadvantages. It is expensive to build, difficult to maintain, and increases the physical size of the hoist. Many designs have a tendency to set up undesirable vibration and chatter. Further, since the load brake is required to absorb and dissipate as heat the energy of the descending load, a large amount of heat is developed, severely limiting the amount of work which the hoist can perform.
The electric holding brake incorporates an electromagnet which, when energized, releases the frictional surfaces otherwise held in contact by means of springs. The coil of the electromagnet is generally connected to one phase of the motor power supply such that when the motor is energized, the brake is released and sets or re-engages when the motor is de-energized.
More recently, several types of hoists have omitted the mechanical load brake and relied on regenerative braking of the induction motor as the second braking means. The phenomena of regenerative braking is a well-known characteristic of induction motors. When the motor is driven above its synchronous speed, it develops braking torque, acts as a generator returning power to the supply. Since the increase in speed required to convert from motor to generator action is not great and motor losses are not significantly different, it can be seen that this approach has significant advantages over the use of a mechanical load brake. However, from a safety standpoint, there is the disadvantage that there is no protection against a simultaneous holding brake and power failure. Should the holding brake fail during operation of the hoist, a free falling load would result when the operator attempted to stop the hoist. An alert operator, however, could keep the load under control by re-energizing the hoist in either the raising or lowering direction. If the power should fail at the same time, there would be no way to prevent the free falling load.
Another disadvantage of eliminating the load brake in prior art hoists has been that in the absence thereof, the holding brake is required to absorb all of the kinetic energy of the hoist and its load each time the hoist is stopped. Since it is a frictional device, it is subject to overheating and wear, thus requiring periodic maintenance and repair.
It has long been recognized that a dual braking system where each brake was effective in the event of power failure was a highly desirable feature. The expensive variable speed hoists used on large overhead cranes and elsewhere, where the inefficiency and other problems of mechanical load brakes could not be tolerated, have used expensive and complicated means to provide this feature, which has been referred to as Off Position Dynamic Braking. On cranes and hoists powered from a direct current source, it is quite simple to connect the direct current drive motor such that it becomes a self-excited generator and thus provides the second source of braking in the absence of external power. On alternating current powered cranes and hoists, the feature could be attained only through the addition of auxiliary equipment. This commonly took the form of an eddy current brake with an auxiliary generator attached to the same shaft to furnish excitation current to the eddy current brake and thus make it independent of an external source of power.
While dynamic braking in any form since it depends on rotation to develop torque, will not hold a suspended load stationary, it does provide controlled lowering at a speed often below the normal lowering speed, and therefore is a relatively safe condition, especially compared to a free falling load.
Still another problem exists with present hoist control systems. It is desirable to minimize the overall heights of the hoist with its load hook in its fully raised position since generally available vertical dimension in workplaces is limited. The vertical height of the hoist, or "headroom" as it is commonly referred to, subtracts from the clearance for moving the material to be handled below the hoist. This means that the hook or lifting device must be capable of being raised as close to the winding means as possible. However, if the lifting means is allowed to contact the winding means or other parts of the hoist while still moving upward, damage to the hoist, even to the extent of causing the load to fall, can result. To prevent this, it is common practice to provide travel limit switches to limit the upward travel of the load hook. These usually take the form of a switch arrangement which first de-energizes the drive motor and sets the holding brake and secondly, if the upward travel continues, as would be the case if the motor brake failed, a second switch actuates to release the brake and re-energizes the motor in the reverse or lowering direction. This is commonly referred to as a plugging type limit switch. Normally, the plugging limit switch stops and reverse the hook motion quite rapidly. There are problems with the system, however. First, there is a time delay between actuation of the switch and actual application of reverse power during which upward travel can continue. Secondly, unless an operator is alert or additional circuitry is added, once the motor is reversed and started downward, the hook will clear the limit switch actuator whereupon, unless the raise control button has been released, the hoist will again start to raise going through alternate raise and lower cycles in a sometimes violent manner until the raise control button is released. Further, it is possible for the motor to fail to respond to the application of reverse power in the manner intended. If, during the raising operation, one phase of the power supply became open circulated, in the case of polyphase motors, the motor would continue to drive the load upward running as a single phase motor. After tripping the plugging limit switch and having power from the single phase reapplied, the motor would not reverse but continue to drive in the upward direction with potentially hazardous results.
The benefits of this invention can be utilized to eliminate the plugging feature from the limit switch, thus reducing the potential hazard thereof. In this case, when the limit switch is tripped, the motor brake is applied and dynamic braking of the motor occurs resulting in faster stopping action. Even should the motor brake fail completely, the dynamic braking effect produces sufficient retarding torque to prevent damage.
It is, therefore, an object of this invention to provide an electric hoist or similar equipment with a means of dynamic braking to assist other braking means in arresting motion thereof without the necessity of auxiliary equipment for the purpose.
It is a further object of this invention to provide an emergency dynamic braking means such that in the event of simultaneous power and holding brake failure, the motor will automatically develop retarding torque sufficient to prevent overspeeding.
It is a further object of this invention to provide more effective and reliable means of arresting motion of the equipment such as at extremes of safe travel thereof.
It is a further object of this invention to provide an electric hoist or similar equipment utilizing single phase drive motors which can be reversed while running without requiring a mechanically actuated switch thus improving reliability thereof.
It is a further object of this invention to provide an electric hoist or similar equipment utilizing single phase drive motor wherein the motor is of the permanent split capacitor type designed to provide less starting torque than a conventional single phase hoist motor such that improved efficiency, reduced starting and running torques are achieved. The lower torque reduces the possibility of dangerously overloading the apparatus and improves control characteristics.
It is a further object of this invention to provide an electric hoist or similar equipment utilizing polyphase drive motors which reduce the tendency for rotation of the drive motor in a direction opposite to that intended when the apparatus is subject to the influence of torque from the driven load in the event the drive is started with an open phase of the power supply.
It is a further object of this invention to provide an electric hoist or similar equipment utilizing polyphase drive motors wherein the apparatus can be operated on a single phase supply at reduced capacity in an emergency or other necessary situations.
In general, the foregoing and other objects will be carried out by providing an electric hoist drive, control and braking apparatus, comprising: an electric hoist motor; a control system for starting, stopping, and controlling the direction and limit of travel of said motor; a capacitor connected to one or more of the motor windings such that upon removal of power from said hoist motor windings, said capacitor causes said hoist motor to develop braking torque upon continued rotation of said motor above a speed substantially below normal operating speed, and said capacitor and said motor windings cooperate to effect and limit starting torque and braking in operative association with the running performance of said motor and the hoist limit control.